The Collisionless Magnetoviscous-Thermal Instability

TL;DR

This paper introduces the collisionless magnetothermal instability (CMTI), a new MHD instability that transports energy and angular momentum in hot, dilute accretion flows around black holes, extending previous fluid-based models to collisionless regimes.

Contribution

It derives a collisionless drift-kinetic equation and characterizes the CMTI, bridging the gap between fluid and collisionless plasma stability analyses in black hole accretion physics.

Findings

Identification of the CMTI as a key instability in collisionless accretion flows.

Demonstration of qualitative similarities between kinetic and fluid instabilities.

Framework for future modeling of hot, dilute accretion disks.

Abstract

It is likely that nearly all central galactic massive and supermassive black holes are nonradiative: their accretion luminosities are orders of magnitude below what can be explained by efficient black hole accretion within their ambient environments. These objects, of which Sagittarius A* is the best-known example, are also dilute (mildly collisional to highly collisionless) and optically thin. In order for accretion to occur, magnetohydrodynamic instabilities must develop that not only transport angular momentum, but also gravitational energy generated through matter infall, outwards. A class of new magnetohydrodynamical fluid instabilities -- the magnetoviscous-thermal instability (MVTI) (Islam12) -- was found to transport angular momentum and energy along magnetic field lines through large (fluid) viscosities and thermal conductivities. This paper describes the collisionless and mildly collisional analogue to the MVTI, the collisional magnetothermal instability (CMTI), that similarly transports energy and angular momentum outwards, expected to be important in describing the flow properties of hot, dilute, and radiatively inefficient accretion flows around black holes. In this paper we derive a form of the collisionless drift-kinetic equation appropriate for accretion physics. We construct a local equilibrium for MHD stability analysis in this differentially rotating disk. We then find and characterize specific instabilities expected to be important in describing the flow properties of hot, dilute, and radiatively inefficient accretion flows around black holes, and show their qualitative similarities to instabilities derived using a fluid formalism (Islam12). We conclude with further work needed in modeling this class of accretion flow.